High-strength hot-rolled steel sheet and method for manufacturing the same

ABSTRACT

There are provided a high-strength hot-rolled steel sheet having high burring formability and a method for manufacturing the high-strength hot-rolled steel sheet. The high-strength hot-rolled steel sheet having high burring formability contains, on a mass percent basis, C: 0.013% or more and less than 0.08%, Si: less than 0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03% or more and 0.15% or less such that C, S, N, and Ti satisfy 0.05≦Ti*&lt;0.1 and C×(48/12)−0.16&lt;Ti* (wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote the amounts (% by mass) of the corresponding elements), the remainder being Fe and incidental impurities, wherein the high-strength hot-rolled steel sheet has a microstructure in which a ferrite phase fraction is more than 90%, a carbide containing Ti is precipitated, and 70% or more of the carbide has a grain size of less than 9 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT InternationalApplication No. PCT/JP2014/000336, filed Jan. 23, 2014, which claimspriority to Japanese Patent Application No. 2013-016455, filed Jan. 31,2013, the disclosures of each of these applications being incorporatedherein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

Aspects of the present invention relate to a high-strength hot-rolledsteel sheet having high burring formability and a method formanufacturing the high-strength hot-rolled steel sheet. A high-strengthhot-rolled steel sheet according to aspects of the present invention maybe used in automotive body components, for example, structural parts,such as members and frames of automotive bodies, and chassis parts, suchas suspensions. However, the present invention is not limited to theseapplications.

BACKGROUND OF THE INVENTION

In recent years, for weight saving of automotive bodies, high-strengthsteel sheets have been actively used as materials for automotive parts.High-strength steel sheets are widely used as automotive structuralparts. For further weight saving of automotive bodies, there is a strongdemand for application of high-strength steel sheets not only tostructural parts but also to chassis parts in which hot-rolled steelsheets are generally used.

Most of automotive parts made of steel sheets are formed intopredetermined shapes by press forming or burring forming. However, ingeneral, higher-strength steel sheets have lower workability. Thus,high-strength steel sheets for use in automotive parts must have highworkability as well as desired strength. For example, because chassisparts are formed by severe processing, both high strength andworkability must be satisfied. In particular, applicability ofhigh-strength steel sheets to these parts and mass productivity of theseparts often depend on burring formability.

Various microstructure control and reinforcement methods have been usedto improve the workability of high-strength hot-rolled steel sheets. Forexample, these methods include use of a complex structure of ductileferrite and hard martensite, use of a bainite microstructure, andprecipitation strengthening of a ferrite microstructure. However,high-strength hot-rolled steel sheets having sufficient workability thatcan be applied to parts to be subjected to severe burring forming, suchas chassis parts, cannot be manufactured in the related art. Thus, thereis a demand for high-strength hot-rolled steel sheet having highworkability.

Patent Literature 1 describes a hot-rolled steel sheet that has acomposition including, on a weight percent basis, C: 0.05% to 0.2%, Si:0.01% to 0.5%, Mn: 0.01% or more and less than 0.5%, P: 0.05% or less,S: 0.01% or less, Al: 0.005% to 0.1%, N: 0.007% or less, and Ti: 0.05%to 0.3%, and has a microstructure including a limited amount ofcementite precipitation. In the technique proposed in Patent Literature1, a decrease in austenite former Mn and extension of an a regionpromote TiC precipitation after hot rolling and before coiling, therebysecuring the strength of the steel sheet due to precipitationstrengthening of TiC. Furthermore, a decrease in the amount of cementitesignificantly improves the hole expandability of the steel sheet. As aresult, 400 to 800 N/mm² high-strength hot-rolled steel sheets havinghigh workability can be manufactured.

Patent Literature 2 describes a technique for manufacturing a hot-rolledsteel sheet having a structure consisting essentially of ferrite havingan average grain size of 5 μm or less by heating, rolling, and coolingsteel having a composition including, on a mass percent basis, C: 0.01%to 0.10%, Si: 1.0% or less, Mn: 2.5% or less, P: 0.08% or less, S:0.005% or less, Al: 0.015% to 0.050%, and Ti: 0.10 to 0.30%, and coilingthe steel sheet at a coiling temperature outside the temperature rangein which TiC is coherently precipitated in a matrix phase. In thetechnique proposed in Patent Literature 2, a single phase structure offerrite having controlled grain size and morphology can impart highstretch-flangeability to the hot-rolled steel sheet without reducing thehigh strength of the hot-rolled steel sheet. Furthermore, coherentprecipitation of TiC in the main phase matrix impairs ductility orstretch-flangeability.

Patent Literature 3 describes a hot-rolled steel sheet that has acomposition including, on a mass percent basis, C: 0.005% or more and0.050% or less, Si: 0.2% or less, Mn: 0.8% or less, P: 0.025% or less,S: 0.01% or less, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% ormore, and 0.10% or less and has a matrix consisting essentially offerrite and a microstructure including finely precipitated Ti carbide.This technique is based on the knowledge that solute strengtheningelements Mn and Si adversely affect stretch-flangeability. Thus, the Mnand Si contents are minimized, and fine Ti carbide is utilized to securethe strength. The technique proposed in Patent Literature 3 can be usedto manufacture a high-strength hot-rolled steel sheet having a tensilestrength of 590 MPa or more and high stretch-flangeability. Furthermore,the addition of B to the composition can suppress coarsening of Ticarbide.

PATENT LITERATURES

-   PTL 1: Japanese Unexamined Patent Application Publication No.    9-209076-   PTL 2: Japanese Unexamined Patent Application Publication No.    2002-105595-   PTL 3: Japanese Unexamined Patent Application Publication No.    2012-26034

SUMMARY OF THE INVENTION

In the technique proposed in Patent Literature 1, a decrease in Mncontent results in a high ferrite transformation temperature andcoarsening of TiC precipitated in the hot-rolled steel sheet. In themanufacture of the hot-rolled steel sheet, TiC is mainly formed duringaustenite→ferrite transformation in cooling and coiling steps after hotrolling. Thus, a high ferrite transformation temperature results inprecipitation of TiC in a high-temperature region and tends to result incoarsening of TiC. When coarsening of TiC occurs in the hot-rolled steelsheet, high burring formability cannot be achieved.

In the technique proposed in Patent Literature 2, the steel sheet iscoiled at a temperature at which TiC is not coherently precipitated inthe main phase matrix in the hot-rolled steel sheet manufacturingprocess. Fine TiC that contributes to increased strength of a steelsheet is not precipitated in a hot-rolled steel sheet manufactured undersuch conditions. Thus, high strength and burring formability cannot besimultaneously satisfied.

With respect to the technique proposed in Patent Literature 3, becauseof the low Mn content, it is difficult to uniformly decrease the ferritetransformation temperature. This results in low production stability andmakes it impossible to finely control the size of Ti carbideprecipitated in the hot-rolled steel sheet. It is also stated that theaddition of B can suppress coarsening of TiC. However, the addition of Btends to elongate ferrite grains, and it is impossible to achieve highductility. Thus, it is difficult to manufacture a hot-rolled steel sheethaving high strength and burring formability.

Burring formability required for mass production of automotive parts isnot referred to in these techniques.

Burring formability of steel sheets has been principally evaluated in ahole-expanding test by a method according to the Japan Iron and SteelFederation standard. However, it is difficult to say that thehole-expanding test accurately simulates a punching process and ahole-expanding process in mass production of automotive parts in actualproduction lines. Thus, there is a problem that steel sheets that areexperimentally shown to have good burring formability according to thestandard often suffer from processing defects in mass production ofautomotive parts.

For example, evaluation of workability in a laboratory alone isinsufficient for mass production of parts. It is necessary to ensureworkability of materials also in consideration of variations inprocessing conditions in mass production. Such problems are notinvestigated in the related art. Thus, the resulting high-strengthhot-rolled steel sheets do not necessarily have desired strength andworkability required for mass production of automotive parts,particularly burring formability (hereinafter also referred to as massproduction burring formability). Techniques that utilize Ti carbide inknown structures consisting essentially of ferrite, for example, thetechniques proposed in Patent Literatures 1 to 3, cannot realize highproduction stability and mass production burring formability ofhigh-strength hot-rolled steel sheets.

As described above, many studies have been made on hot-rolled steelsheets having high stretch-flangeability (burring formability). However,high-strength hot-rolled steel sheets that satisfy mass productionburring formability, that is, severe burring formability required inactual automotive part production lines are not necessarily manufacturedin the related art.

It is an aim of aspects of the present invention to advantageouslyprovide a high-strength hot-rolled steel sheet having a tensile strength(TS) of 540 MPa or more and having high burring formability,particularly high mass production burring formability, and a method formanufacturing the high-strength hot-rolled steel sheet.

The “mass production burring formability” herein is evaluated as aburring ratio measured in a hole-expanding test using a 60-degreeconical punch after punching with a 50-mmφ punch (clearance of stamping:30%) and is different from burring formability evaluated as a λ valuedetermined by a known hole-expanding test method, for example, ahole-expanding test method according to the Japan Iron and SteelFederation standard.

Solution to Problem

The present inventors studied methods for evaluating Mass productionburring formability. Burring formability has been evaluated as a λvalue, for example, measured by a hole-expanding test method accordingto the Japan Iron and Steel Federation standard. In this case, the punchdiameter is 10 mmφ. However, the present inventors found that burringformability in actual mass production settings for parts is notcorrelated with the λ value measured in laboratories according to theJapan Iron and Steel Federation standard. It was found that burringformability evaluated in a new hole-expanding test that includeshole-expanding using a 60-degree conical punch after punching with a50-mmφ punch (clearance of stamping: 30%) is closely correlated withmass production punchability and mass production burring formability.

The present inventors also extensively studied various factors thatcontribute to high strength and workability, particularly massproduction burring formability, of hot-rolled steel sheets by evaluatingmass production burring formability in the new hole-expanding test.

Moreover, extensive studies have been made on means for improving massproduction burring formability of a hot-rolled steel sheet based on astructure consisting essentially of a ductile ferrite phase whilereinforcing the hot-rolled steel sheet, with consideration given to allthe precipitates that can be precipitated in the hot-rolled steel sheet,such as nitrides, sulfides, carbides, and complex precipitates thereof(for example, carbonitride).

It was found that a hot-rolled steel sheet having a tensile strength of540 MPa or more that satisfies severe mass production burringformability required in actual automotive part production lines can bemanufactured by optimizing the balance between the amount of C in thehot-rolled steel sheet and the amount of Ti (Ti*) that contributes tothe formation of carbide and increasing the percentage of carbide havinga grain size of less than 9 nm in carbide precipitated in the hot-rolledsteel sheet. It was also found that mass production burring formabilitycan be further improved by controlling the size of not only carbide butalso all the precipitates that can be precipitated in a hot-rolled steelsheet (nitrides, sulfides, carbides, and complex precipitates thereof).

The present inventors also studied means for achieving a desirable sizeof precipitates that are precipitated in a hot-rolled steel sheet(nitrides, sulfides, carbides, and complex precipitates thereof), thatis, a size required to impart desired strength (e.g., tensile strengthof 540 MPa or more) and good mass production burring formability to thehot-rolled steel sheet. As a result, it was found that it is desirableto properly control the Mn content and C, S, N, and Ti contents of ahot-rolled steel sheet and optimize the hot rolling conditions and thecooling and coiling conditions after hot rolling.

The following are non-limiting embodiments of the present invention.

[1] A high-strength hot-rolled steel sheet, containing: on a masspercent basis, C: 0.013% or more and less than 0.08%, Si: less than0.5%, Mn: more than 0.8% and less than 1.2%, P: 0.05% or less, S: 0.005%or less, N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03% or more and0.15% or less such that C, S, N, and Ti satisfy the following formulae(1) and (2), the remainder being Fe and incidental impurities, whereinthe high-strength hot-rolled steel sheet has a microstructure in which aferrite phase fraction is more than 90%, a carbide containing Ti isprecipitated, and 70% or more of the carbide has a grain size of lessthan 9 nm.

0.05≦Ti*<0.1  (1)

C×(48/12)−0.16<Ti*  (2)

wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote theamounts (% by mass) of the corresponding elements.[2] The high-strength hot-rolled steel sheet according to [1], wherein50% by mass or more of Ti is precipitated as precipitates containing Tihaving a grain size of less than 20 nm.[3] The high-strength hot-rolled steel sheet according to [1] or [2],further containing at least one of V: 0.002% or more and 0.1% or lessand Nb: 0.002% or more and 0.1% or less on a mass percent basis.[4] The high-strength hot-rolled steel sheet according to any one of [1]to [3], further containing at least one of Cu: 0.005% or more and 0.2%or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002% or more and0.2% or less, and Mo: 0.002% or more and 0.2% or less on a mass percentbasis.[5] The high-strength hot-rolled steel sheet according to any one of [1]to [4], further containing B: 0.0002% or more and 0.003% or less on amass percent basis.[6] The high-strength hot-rolled steel sheet according to any one of [1]to [5], further containing at least one of Ca: 0.0002% or more and0.005% or less and REM: 0.0002% or more and 0.03% or less on a masspercent basis.[7] A method for manufacturing a high-strength hot-rolled steel sheet,including: heating steel having a composition according to any one of[1] and [3] to [6] to 1100° C. or more, hot-rolling the steel at afinish-rolling temperature of (Ar₃+20° C.) or more and at a totalreduction ratio of 60% or less at last two finish rolling stands,cooling the hot-rolled steel sheet at an average cooling rate of 40°C./s or more, and coiling the hot-rolled steel sheet at a coilingtemperature in the range of 560° C. to 720° C.[8] A method for manufacturing a high-strength hot-rolled steel sheet,comprising: heating steel having a composition according to any one of[1] and [3] to [6] to 1100° C. or more, hot-rolling the steel at afinish-rolling temperature of (Ar₃+20° C.) or more and at a totalreduction ratio of 60% or less at last two finish rolling stands,cooling the hot-rolled steel sheet at an average cooling rate of 40°C./s or more, coiling the hot-rolled steel sheet at a coilingtemperature in the range of 500° C. to 660° C., annealing the hot-rolledsteel sheet at a soaking temperature of 750° C. or less after pickling,and plating the hot-rolled steel sheet by immersing the hot-rolled steelsheet in a molten zinc bath.[9] The method for manufacturing a high-strength hot-rolled steel sheetaccording to [8], wherein the plating treatment is followed by alloyingtreatment.

Aspects of the present invention provide a high-strength hot-rolledsteel sheet having a tensile strength of 540 MPa or more and highburring formability such that the high-strength hot-rolled steel sheetcan be subjected to processing in mass production of automotive parts.Thus, in one embodiment, a high-strength hot-rolled steel sheet can beapplied to structural parts, such as members and frames of automotivebodies, and chassis parts, such as suspensions. Embodiments of thepresent invention may contribute greatly to weight saving of theseparts.

Aspects of the present invention can provide a hot-rolled steel sheethaving a tensile strength of 540 MPa or more and good mass productionburring formability. Thus, the high-strength hot-rolled steel sheet canbe applied not only to automotive parts but also to other applications.Thus, the present invention has industrially advantageous effects.

DETAILED DESCRIPTION OF THE INVENTION

Non-limiting embodiments of the present invention will be furtherdescribed below.

A high-strength hot-rolled steel sheet according to one embodimentcontains, on a mass percent basis, C: 0.013% or more and less than0.08%, Si: less than 0.5%, Mn: more than 0.8% and less than 1.2%, P:0.05% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.1% or less,and Ti: 0.03% or more and 0.15% or less such that C, S, N, and Tisatisfy the following formulae (1) and (2), the remainder being Fe andincidental impurities, wherein the high-strength hot-rolled steel sheethas a microstructure in which a ferrite phase fraction is more than 90%,a carbide containing Ti is precipitated, and 70% or more of the carbidehas a grain size of less than 9 nm.

0.05≦Ti*<0.1  (1)

C×(48/12)−0.16<Ti*  (2)

wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote theamounts (% by mass) of the corresponding elements.

First, examples of the composition of a hot-rolled steel sheet accordingto aspects of the present invention will be described below. Unlessotherwise specified, the percentages of the components are on a masspercent basis.

C: 0.013% or More and Less than 0.08%

C is an important element that forms an appropriate carbide in ahot-rolled steel sheet and secures the strength of the steel sheet. Inorder to achieve the desired tensile strength (540 MPa or more), the Ccontent is 0.013% or more. However, a C content of 0.08% or more resultsin poor workability and undesired burring formability of a hot-rolledsteel sheet. Thus, the C content is 0.013% or more and less than 0.08%,preferably 0.03% or more and 0.07% or less.

Si: Less than 0.5%

A Si content of 0.5% or more results in very low surface quality of ahot-rolled steel sheet, which adversely affects fatigue characteristics,chemical conversion treatability, and corrosion resistance. Si increasesthe ferrite transformation temperature and thereby adversely affects theformation of fine precipitates. Thus, the Si content is less than 0.5%,preferably 0.001% or more and less than 0.1%, more preferably 0.001% ormore and less than 0.05%.

Mn: More than 0.8% and Less than 1.2%

Mn is an important element. Mn significantly influences precipitation ofa carbide containing Ti, through control of austenite-to-ferritetransformation temperatures.

In the case of a hot-rolled steel sheet containing Ti, a carbidecontaining Ti is mainly precipitated by austenite→ferrite transformationin cooling and coiling steps after finish rolling in a hot-rolled steelsheet manufacturing process. Among carbides precipitated in a hot-rolledsteel sheet, fine carbide contributes to high strength of the hot-rolledsteel sheet, but coarse carbide does not contribute to high strength andadversely affects the workability of the hot-rolled steel sheet.

A high austenite-ferrite transformation temperature results inprecipitation of a carbide containing Ti in a high-temperature regionand consequently coarsening of the carbide containing Ti. Thus, in orderto decrease the size of the carbide containing Ti, it is preferable todecrease the austenite-ferrite transformation temperature.

Mn is an element that has an effect of decreasing the austenite-ferritetransformation temperature. A Mn content of 0.8% or less results in aninsufficient decrease in the austenite-ferrite transformationtemperature. As a result, it is difficult for a carbide containing Ti tohave a desirable size. Thus, it is difficult to provide a high-strengthhot-rolled steel sheet having high mass production burring formability.On the other hand, a Mn content of 1.2% or more results in saturation ofthe effect and increased costs. An excessively high Mn content of 1.2%or more also results in increased Mn segregation in the central portionin the thickness direction. This center segregation impairs a punchedsurface before burring forming and is therefore responsible for low massproduction burring formability. Thus, the Mn content is more than 0.8%and less than 1.2%, preferably more than 0.8% and less than 1.0%.

P: 0.05% or Less

P promotes low workability of a hot-rolled steel sheet due tosegregation. Thus, the P content is 0.05% or less, preferably 0.001% ormore and 0.03% or less. In the case of a galvanized steel sheet formedby galvanizing treatment of a hot-rolled steel sheet, the P content ispreferably 0.005% or more, more preferably 0.01% or more, still morepreferably 0.015% or more, in terms of platability.

S: 0.005% or Less

S forms a sulfide and decreases the workability of a hot-rolled steelsheet. Thus, the S content is 0.005% or less, preferably 0.0001% or moreand 0.003% or less, more preferably 0.0001% or more and 0.0015% or less.

N: 0.01% or Less

An excessively high N content of more than 0.01% results in theformation of a large amount of nitride in a hot-rolled steel sheetmanufacturing process, low hot ductility, and very low burringformability of a hot-rolled steel sheet due to coarsening of nitride.Thus, the N content is 0.01% or less, preferably 0.0001% or more and0.006% or less, more preferably 0.0001% or more and 0.004% or less.

Al: 0.1% or Less

Al is a useful element as a deoxidizing agent for steel. However, an Alcontent of more than 0.1% makes casting of steel difficult and resultsin a large amount of residual inclusion in steel and low surface qualityand workability of a hot-rolled steel sheet. Thus, the Al content is0.1% or less, preferably 0.001% or more and 0.06% or less.

Ti: 0.03% or More and 0.15% or Less

Ti is an important element in the present invention. Ti forms finecarbide and contributes to increased strength of a hot-rolled steelsheet. In order to achieve the desired strength of a hot-rolled steelsheet (tensile strength of 540 MPa or more), the Ti content is 0.03% ormore. However, a Ti content of more than 0.15% tends to result in theremaining coarse carbide in a hot-rolled steel sheet. Coarse carbide hasno strength increasing effect and greatly impairs the workability,toughness, and weldability of a hot-rolled steel sheet. Thus, the Ticontent is 0.03% or more and 0.15% or less, preferably 0.04% or more and0.12% or less.

A hot-rolled steel sheet according to one embodiment contains C, S, N,and Ti in the ranges described above so as to satisfy the formulae (1)and (2). The formulae (1) and (2) are desirably satisfied in order toachieve high strength and good mass production burring formability of ahot-rolled steel sheet and are beneficial indicators. In the formulae(1) and (2), Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N, and Ti denote theamounts (%) of the corresponding elements.

0.05≦Ti*<0.1  (1)

As described below, in one embodiment, a predetermined amount of Ti isadded to steel, and carbide in steel is dissolved by heating before hotrolling. A carbide containing Ti is mainly precipitated during coilingafter hot rolling. However, Ti added to steel does not entirelycontribute to the formation of carbide and is partly consumed by formingnitride or sulfide. This is because Ti is likely to form nitride orsulfide rather than carbide in a higher temperature region than thecoiling temperature. Thus, Ti forms nitride or sulfide before thecoiling step in the production of a hot-rolled steel sheet. Thus, theminimum amount of Ti that can contribute to the formation of carbide outof Ti added to steel can be represented by Ti*(=Ti−N×(48/14)−S×(48/32)).

A hot-rolled steel sheet cannot have the desired strength (tensilestrength of 540 MPa or more) at Ti* of less than 0.05. Thus, in oneembodiment, Ti* is 0.05 or more, preferably 0.055 or more. However, ahot-rolled steel sheet has a tensile strength of 700 MPa or more at Ti*of 0.1 or more. A hot-rolled steel sheet having such excessively highstrength has poor workability. Furthermore, an excessively high Ti*results in coarsening of precipitates containing Ti, such as Ti carbide,Ti carbonitride, Ti nitride, and Ti sulfide, which results in low massproduction burring formability. Thus, in in one embodiment, Ti* is lessthan 0.1.

C×(48/12)−0.16<Ti*  (2)

The formula (2) represents the relationship between the amount of Ti*and the amount of C. An excessively higher amount of C than the amountof Ti* results in coarsening of Ti carbide and Ti carbonitride,precipitation of coarse cementite or pearlite, and a significantreduction in the workability of a hot-rolled steel sheet, such as massproduction burring formability. In one embodiment, C×(48/12)−0.16<Ti*,preferably C×(48/12)−0.15<Ti*. However, an excessively high amount of Tithan the amount of C tends to result in low toughness and weldability ofa hot-rolled steel sheet. Thus, Ti*<C×(48/12)+0.08 is preferred. Morepreferably, Ti*<C×(48/12)+0.06.

The following are base components of a hot-rolled steel sheet accordingto non-limiting embodiments of the present invention. A hot-rolled steelsheet according to one embodiment may contain at least one of V: 0.002%or more and 0.1% or less and Nb: 0.002% or more and 0.1% or less, ifdesirable.

V and Nb are effective in decreasing the size of crystal grains andimproving the toughness of a hot-rolled steel sheet. Thus, V and Nb maybe added as desired. Part of added V and/or Nb is precipitated togetherwith Ti as fine complex carbide or complex precipitates and therebycontributes to precipitation strengthening. In order to produce such aneffect, the V content is preferably 0.002% or more, and the Nb contentis preferably 0.002% or more. However, a V or Nb content of more than0.1% is not worth the cost. Thus, the V content is preferably 0.002% ormore and 0.1% or less, more preferably 0.002% or more and 0.08% or less.The Nb content is preferably 0.002% or more and 0.1% or less, morepreferably 0.002% or more and 0.08% or less.

A hot-rolled steel sheet according to one embodiment may contain atleast one of Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and0.2% or less, Cr: 0.002% or more and 0.2% or less, and Mo: 0.002% ormore and 0.2% or less, if desirable.

Cu and Ni are elements that contribute to high strength of a hot-rolledsteel sheet and may be added, if desirable. In order to produce such aneffect, the Cu content is preferably 0.005% or more, and the Ni contentis preferably 0.005% or more. However, a Cu or Ni content of more than0.2% may result in surface layer cracking during hot rolling in theproduction of a hot-rolled steel sheet. Thus, the Cu content ispreferably 0.005% or more and 0.2% or less, more preferably 0.005% ormore and 0.1% or less. The Ni content is preferably 0.005% or more and0.2% or less, more preferably 0.005% or more and 0.15% or less.

Cr and Mo are carbide formation elements, contribute to high strength ofa hot-rolled steel sheet, and may be added, if desirable. In order toproduce such an effect, the Cr content is preferably 0.002% or more, andthe Mo content is preferably 0.002% or more. However, a Cr or Mo contentof more than 0.2% is not worth the cost. Thus, the Cr content ispreferably 0.002% or more and 0.2% or less, more preferably 0.002% ormore and 0.1% or less. The Mo content is preferably 0.002% or more and0.2% or less, more preferably 0.002% or more and 0.1% or less.

A hot-rolled steel sheet according to one embodiment may contain B:0.0002% or more and 0.003% or less, if desirable.

B is an element that retards austenite-ferrite transformation of steel.B decreases the precipitation temperature of a carbide containing Ti bysuppressing austenite-ferrite transformation and contributes to areduction in the size of the carbide. In order to produce such aneffect, the B content is preferably 0.0002% or more. However, a Bcontent of more than 0.003% results in a strong bainite transformationeffect of B, making it difficult for a hot-rolled steel sheet to have astructure consisting essentially of a ferrite phase. The B content ispreferably 0.0002% or more and 0.003% or less, more preferably 0.0002%or more and 0.002% or less.

A hot-rolled steel sheet according to one embodiment may contain atleast one of Ca: 0.0002% or more and 0.005% or less and REM: 0.0002% ormore and 0.03% or less, if desirable.

Ca and REM are elements that are effective in morphology control of aninclusion in steel and contribute to improved workability of ahot-rolled steel sheet. In order to produce such an effect, the Cacontent is preferably 0.0002% or more, and the REM content is preferably0.0002% or more. However, a Ca content of more than 0.005% or a REMcontent of more than 0.03% may result in an increased inclusion in steeland low workability of a hot-rolled steel sheet. Thus, the Ca content ispreferably 0.0002% or more and 0.005% or less, more preferably 0.0002%or more and 0.003% or less. The REM content is preferably 0.0002% ormore and 0.03% or less, more preferably 0.0002% or more and 0.003% orless.

The remainder may be Fe and incidental impurities. Examples of theincidental impurities include W, Co, Ta, Sn, Sb, Zr, and O. The amountof each of the incidental impurities may be 0.1% or less.

Next, the microstructure of a hot-rolled steel sheet according tonon-limiting embodiments of the present invention will be describedbelow.

A hot-rolled steel sheet according to one embodiment has amicrostructure in which the ferrite phase fraction is more than 90%, acarbide containing Ti is precipitated, and 70% or more of the carbidehas a grain size of less than 9 nm. A hot-rolled steel sheet preferablyhas a microstructure in which 50% by mass or more of Ti is precipitatedas precipitates having a grain size of less than 20 nm.

Ferrite Phase Fraction: More than 90%

The burring formability of a hot-rolled steel sheet can be effectivelyimproved when the hot-rolled steel sheet has a microstructure includinga ductile ferrite phase. In order to achieve a preferable high massproduction burring formability, the ferrite fraction in themicrostructure of a hot-rolled steel sheet is more than 90%, preferablymore than 94%, more preferably more than 96% by area. It is desirablethat the ferrite grains have a polygonal shape from the perspective ofburring formability. It is also desirable that the ferrite grain size beas small as possible. The hot-rolled steel sheet preferably has a singlephase structure of ferrite in terms of burring formability. In order toimprove punchability, the ferrite fraction is preferably 99% by area orless.

A hot-rolled steel sheet according to one embodiment may have amicrostructure other than the ferrite phase, such as cementite,pearlite, bainite, martensite, and/or retained austenite. Although thesemicrostructures in the hot-rolled steel sheet impair burringformability, these microstructures may constitute approximately lessthan 10%, preferably less than 6%, more preferably less than 4% by areain total.

Carbide Containing Ti

In one embodiment, the desired strength (tensile strength of 540 MPa ormore) of a hot-rolled steel sheet is achieved by precipitation of acarbide containing Ti in the hot-rolled steel sheet. The carbidecontaining Ti is mainly precipitated carbide resulting fromaustenite→ferrite transformation in the cooling and coiling steps afterfinish rolling in a hot-rolled steel sheet manufacturing process.

In order to make the maximum use of the precipitation strengtheningeffect and optimize the balance between strength and workability (massproduction burring formability), it may be preferable to reduce the sizeof a carbide containing Ti precipitated in a hot-rolled steel sheet. Asa result of extensive studies, the present inventors found that 70% bynumber or more, preferably 80% or more, of a carbide containing Ti has agrain size of less than 9 nm in order to achieve the desirablecharacteristics. The “carbide containing Ti” includes complex carbidescontaining Ti and at least one of Nb, V, Cr, and Mo as well as Ticarbide.

Precipitate Containing Ti

The size of precipitates containing Ti may be controlled to furtherimprove the mass production burring formability of a hot-rolled steelsheet.

As described above, in the case of a hot-rolled steel sheet made ofsteel containing Ti, in addition to carbide (carbide containing Ti) thatcontributes to high strength of a hot-rolled steel sheet, nitride,carbonitride, and sulfide containing Ti are precipitated. In theproduction of a hot-rolled steel sheet, these nitride, carbonitride, andsulfide are precipitated faster than carbide containing Ti. Thus,nitride, carbonitride, and sulfide containing Ti are precipitated in ahigher temperature range than carbide and are therefore easily coarsenedand tend to impair mass production burring formability.

As a result of extensive studies, the present inventors found that thecontrol of the amount and size of these precipitates is very effectivein improving mass production burring formability. Preferably, 50% bymass or more, more preferably 60% by mass or more and 85% by mass orless, of Ti in a hot-rolled steel sheet is preferably precipitated asprecipitates containing Ti having a grain size of less than 20 nm. Theprecipitates containing Ti having a grain size of less than 20 nm aremostly carbide containing Ti and also include nitride, carbonitride, andsulfide containing Ti.

The precipitates containing Ti may be precipitates of Ti carbide, Tinitride, Ti sulfide, and/or Ti carbonitride, and/or complexprecipitates, such as complex carbide, complex nitride, complex sulfide,and/or complex carbonitride containing Ti and at least one of Nb, V, Cr,and Mo.

Even when precipitates having a grain size of 20 nm or more out ofprecipitates containing Ti are precipitated, it is surmised that aproper amount of precipitates contribute to improved punchability beforeburring forming and consequently contribute to improved burringformability.

Formation of a coated layer on a surface of a hot-rolled steel sheet inorder to impart corrosion resistance does not reduce the advantages ofthe present invention. The type of coated layer formed on a surface of ahot-rolled steel sheet is not particularly limited and may be galvanicelectroplating or hot-dip plating. The hot-dip plating may be hot-dipgalvanization. The coated layer may also be galvannealed steel, whichwas subjected to alloying treatment after plating.

Methods for manufacturing a hot-rolled steel sheet will be describedbelow with reference to non-limiting embodiments.

One embodiment includes heating steel having the composition describedabove to 1100° C. or more, hot-rolling the steel at a finish-rollingtemperature of (Ar₃+20° C.) or more and at a total reduction ratio of60% or less at last two finish rolling stands, cooling the hot-rolledsteel sheet at an average cooling rate of 40° C./s or more, and coilingthe hot-rolled steel sheet at a coiling temperature in the range of 560°C. to 720° C.

The steel may be melted by any method, for example, in a converter,electric furnace, or induction furnace. After that, secondary smeltingis preferably performed with vacuum degassing equipment. Subsequentcasting is preferably performed in a continuous casting process in termsof productivity and quality. A blooming method can also be used. A slab(steel) to be casted may be a general slab having a thickness in therange of approximately 200 to 300 mm or a thin slab having a thicknessof approximately 30 mm. In the case of a thin slab, rough rolling may beomitted. A slab after casting may be subjected to hot direct rolling ormay be subjected to hot rolling after reheating in a furnace.

Steel Heating Temperature: 1100° C. or More

Steel thus produced is subjected to hot rolling. In one embodiment, itis desirable to heat the steel (slab) before hot rolling and redissolvecarbide in the steel. At a steel heating temperature of less than 1100°C., carbide is not redissolved in the steel, and desirable fine carbidecannot be formed in the cooling and coiling steps after hot rolling.Thus, the steel heating temperature is 1100° C. or more, preferably1200° C. or more, more preferably 1240° C. or more.

However, an excessively high steel heating temperature results inexcessively accelerated oxidation of the surface of a steel sheet andvery poor surface quality and adversely affects the workability of ahot-rolled steel sheet. Thus, the steel heating temperature ispreferably 1350° C. or less.

After heating of steel, the steel is subjected to hot rolling, which iscomposed of rough rolling and finish rolling. The rough rollingconditions are not particularly limited. As described above, when steelis a thin slab, rough rolling may be omitted. In the finish rolling, thefinish-rolling temperature is (Ar₃+20° C.) or more, and the totalreduction ratio at last two stands of a finish rolling mill is 60% orless.

Finish-Rolling Temperature: (Ar₃+20° C.) or More

At a finish-rolling temperature of less than (Ar₃+20° C.),austenite→ferrite transformation in the cooling and coiling steps afterhot rolling is ferrite transformation from unrecrystallized austenitegrains. In such a case, desired fine carbide cannot be formed, and ahot-rolled steel sheet cannot have a desirable strength (e.g., tensilestrength of 540 MPa or more). Thus, the finish-rolling temperature is(Ar₃+20° C.) or more, preferably (Ar₃+40° C.) or more. However, anexcessively high finish-rolling temperature results in coarsening ofcrystal grains and adversely affects the punchability of a hot-rolledsteel sheet. Thus, the finish-rolling temperature is (Ar₃+140° C.) orless.

The Ar₃ transformation point herein refers to a transformationtemperature at a change point of a thermal expansion curve measured in athermecmastor test (thermo-mechanical simulation test) at a cooling rateof 5° C./s.

Total Reduction Ratio at Last Two Finish Rolling Stands: 60% or Less

When the total reduction ratio at last two finish rolling stands exceeds60%, this results in increased residual strain and accelerates ferritetransformation from unrecrystallized austenite grains. Thus, the totalreduction ratio at last two stands of a finish rolling mill is 60% orless, preferably 50% or less.

Average Cooling Rate: 40° C./s or More

When the average cooling rate in cooling after hot rolling is less than40° C./s, this results in a high ferrite transformation temperature. Asa result, carbide is precipitated in a high temperature region, desiredfine carbide cannot be formed, and a hot-rolled steel sheet cannot havethe desired strength (e.g., tensile strength of 540 MPa or more). Thus,the average cooling rate is 40° C./s or more, preferably 50° C./s ormore. However, at an excessively high average cooling rate, it may bedifficult to achieve the desired ferrite microstructure. Thus, theaverage cooling rate is 150° C./s or less.

The average cooling rate herein refers to the average cooling ratebetween the finish-rolling temperature and the coiling temperature.

In one embodiment, a carbide containing Ti is precipitated in a periodfrom immediately before coiling to the beginning of the coiling step bydecreasing the ferrite transformation temperature so as to be close tothe coiling temperature at the average cooling rate. This can preventprecipitation and coarsening of the carbide containing Ti in a hightemperature region. Thus, the resulting hot-rolled steel sheet cancontain precipitated fine carbide.

Coiling Temperature: 560° C. to 720° C.

As described above, in one embodiment, fine carbide containing Ti ismainly precipitated in a period from immediately before coiling to thebeginning of the coiling step. Thus, in order to precipitate a largeamount of fine carbide containing Ti, the coiling temperature iscontrolled in a temperature range suitable for precipitation of thecarbide containing Ti. At a coiling temperature of less than 560° C. ormore than 720° C., fine carbide that contributes to high strength ofsteel is not sufficiently precipitated, and the hot-rolled steel sheetmay not have a desirable strength. For these reasons, the coilingtemperature ranges from 560° C. to 720° C., preferably 600° C. to 700°C.

In one embodiment, a hot-rolled steel sheet after coiling may besubjected to pickling and annealing treatment and then to platingtreatment by immersion in a molten zinc bath. After the platingtreatment, the hot-rolled steel sheet may be subjected to alloyingtreatment. When the plating treatment is performed, the coilingtemperature ranges from 500° C. to 660° C., and the soaking temperaturefor the annealing treatment is 750° C. or less.

Coiling Temperature: 500° C. to 660° C.

A higher coiling temperature facilitates the formation of an internaloxidation layer in a hot-rolled steel sheet. The internal oxidationlayer may promote plating defects. For example, a coiling temperature ofmore than 660° C. results in low plating quality. On the other hand, alow coiling temperature is preferred in order to prevent platingdefects. However, a coiling temperature of less than 500° C. results ininsufficient precipitation of a carbide containing Ti, and a hot-rolledsteel sheet may not have a desirable strength. Thus, when platingtreatment is performed after coiling, the coiling temperature rangesfrom 500° C. to 660° C., preferably 500° C. to 600° C.

Soaking Temperature: 750° C. or Less

As described above, when the coiling temperature is lowered for platingtreatment, fine carbide that contributes to high strength of ahot-rolled steel sheet (carbide containing Ti) may be insufficientlyprecipitated during coiling. Thus, in one embodiment, the desiredstrength (tensile strength of 540 MPa or more) of a hot-rolled steelsheet after plating treatment is achieved by precipitating fine carbide(carbide containing Ti) during annealing treatment before the platingtreatment. When the soaking temperature for annealing treatment exceeds750° C., precipitated carbide (carbide containing Ti) is coarsened, anda hot-rolled steel sheet has low strength. Thus, the soaking temperaturefor annealing treatment is 750° C. or less, preferably 720° C. or less.In order to promote precipitation of fine carbide (carbide containingTi), the soaking temperature for annealing treatment is preferably 600°C. or more. The holding time at the soaking temperature preferablyranges from 10 to 1000 seconds.

After the annealing treatment, the steel sheet is immersed in a hot-dipgalvanizing bath to form a hot-dip galvanized layer on the surface ofthe steel sheet. After immersion in the hot-dip galvanizing bath, thesteel sheet may be subjected to alloying treatment. The annealingtreatment and plating treatment are preferably performed in a continuoushot-dip galvanizing line.

The type of plating is not limited to hot-dip galvanization orgalvannealing described above and may be electrogalvanizing.

The plating treatment conditions, the alloying treatment conditions, andother manufacturing conditions are not particularly limited and may begeneral conditions.

EXAMPLES

Steel slabs (Nos. A to P) containing the components listed in Table 1and having the Ar₃ transformation point listed in Table 1 were heated toa temperature in the range of 1180° C. to 1290° C., and hot-rolled steelsheets (Nos. 1 to 22) were formed under the hot-rolling conditionslisted in Table 2. The hot-rolled steel sheets had a thickness in therange of 2.0 to 4.5 mm. The Ar₃ transformation points listed in Table 1were determined as described above. Part of the hot-rolled steel sheets(Nos. 3, 4, 9 to 11, 16, 18, and 19) were subjected to pickling and werethen subjected to annealing treatment at a soaking temperature listed inTable 2 and hot-dip galvanizing treatment in a hot-dip galvanizationline. In the hot-dip galvanizing treatment, each of the hot-rolled steelsheets subjected to the annealing treatment was immersed in agalvanizing bath (0.1% by mass Al—Zn) at 480° C., and a hot-dipgalvanized layer was formed on both faces of the steel sheet at 45 g/m².Part of the hot-rolled steel sheets (Nos. 9 to 11, 16, 18, and 19) weresubjected to the hot-dip galvanizing treatment and alloying treatment.The alloying treatment temperature was 520° C.

Test specimens were taken from the hot-rolled steel sheets (Nos. 1 to22) and were subjected to microstructure observation, a tensile test,and a hole-expanding test. The microstructure observation method andvarious test methods were as follows:

(i) Microstructure Observation Ferrite Phase Fraction

Scanning electron microscope (SEM) test specimens were taken from thehot-rolled steel sheets. A vertical cross section of each of the testspecimens parallel to the rolling direction was polished and wassubjected to nital etching. SEM photographs were taken in 10 visualfields at a quarter thickness in the depth direction and at amagnification ratio of 3000. A ferrite phase and a non-ferrite phasewere separated by image analysis. The fraction of each of the phases(area fraction) was determined.

Carbide Containing Ti

Thin film samples were prepared from the hot-rolled steel sheets (at aquarter thickness in the depth direction). Photographs were taken in 10visual fields with a transmission electron microscope at a magnificationratio of 200,000.

The number of carbide grains containing Ti (N₀) was determined from thephotographs. The grain size of each carbide grain containing Ti wasdetermined as an equivalent circle diameter by image processing. Thenumber of carbide grains having a grain size of less than 9 nm (N₁) outof the carbide grains containing Ti was determined. The ratio of thenumber of carbide grains having a grain size of less than 9 nm to thenumber of carbide grains containing Ti (N₁/N₀×100(%)) was calculatedfrom these numbers (N₀ and N₁).

Precipitates Containing Ti

Precipitates were extracted from the hot-rolled steel sheets byconstant-current electrolysis using an AA electrolyte solution (anethanol solution of acetylacetone-tetramethylammonium chloride). Theextract was passed through a filter having a pore size 20 nm.Precipitates having a size of less than 20 nm were separated in thismanner, and the amount of Ti in the precipitates having a size of lessthan 20 nm was measured by inductively-coupled plasma optical emissionspectrometry (ICP). The ratio (percentage) of Ti in the precipitateshaving a size of less than 20 nm was determined by dividing the amountof Ti in the precipitates having a size of less than 20 nm by the amountof Ti in the hot-rolled steel sheet.

(ii) Tensile Test

Three JIS No. 5 test pieces for tensile test were taken from each of thehot-rolled steel sheets such that the tensile direction is a directionperpendicular to the rolling direction. The tensile strength and totalelongation were measured in a tensile test (strain rate: 10 mm/min)according to JIS Z 2241 (2011). Each of the hot-rolled steel sheets weresubjected to measurements three times in the tensile test. The tensilestrength (TS) and total elongation (El) were averages of the threemeasurements.

(iii) Hole-Expanding Test (Evaluation of Mass Production BurringFormability)

Test specimens (size: 150 mm×150 mm) were taken from the hot-rolledsteel sheets. A hole having an initial diameter d₀ was formed in each ofthe test specimens by punching with a 50-mmφ punch (clearance ofstamping: 30%). The hole was then expanded by inserting a conical punchhaving a vertex angle of 60 degrees into the hole from the punched side.A hole diameter d₁ at which a crack penetrated the steel sheet (testspecimen) in the thickness direction was measured. The burring ratio (%)was calculated using the following equation.

Burring ratio (%)={(d ₁ −d ₀)/d ₀}×100

A burring ratio of 60% or more was considered to be high mass productionburring formability.

Table 3 shows the results.

TABLE 1 Chemicalcomposition (mass %) Ar₃ Formulae (1) Steel C Si Mn P SAl N Ti Others Ti* (° C.) and (2)*1 Remarks A 0.031 0.11 1.19 0.0450.0035 0.049 0.0018 0.062 — 0.051 875 ∘ Inventive steel B 0.042 0.451.04 0.029 0.0004 0.019 0.0036 0.072 V: 0.009 0.059 878 ∘ Inventivesteel C 0.039 0.03 0.84 0.021 0.0017 0.039 0.0033 0.088 B: 0.0009 0.074861 ∘ Inventive steel D 0.030 0.19 0.81 0.033 0.0005 0.033 0.0023 0.089— 0.080 861 ∘ Inventive steel E 0.049 0.01 0.99 0.009 0.0009 0.0440.0037 0.096 — 0.082 882 ∘ Inventive steel F 0.034 0.03 1.08 0.0120.0014 0.023 0.0072 0.085 Ni: 0.19, REM: 0.001 0.058 844 ∘ Inventivesteel G 0.023 0.48 0.96 0.008 0.0026 0.067 0.0011 0.104 Cr: 0.04, Ca:0.0014 0.096 877 ∘ Inventive steel H 0.055 0.03 0.88 0.011 0.0009 0.0410.0044 0.088 Mo: 0.07 0.072 845 ∘ Inventive steel I 0.019 0.02 0.850.017 0.0011 0.045 0.0055 0.118 Nb: 0.011 0.097 869 ∘ Inventive steel J0.064 0.12 0.93 0.015 0.0006 0.039 0.0014 0.105 Cu: 0.04, Ni: 0.08 0.099847 ∘ Inventive steel K 0.025 0.03 0.31 0.011 0.0031 0.041 0.0050 0.076B: 0.0020 0.054 878 ∘ Comparative steel L 0.061 0.04 1.37 0.019 0.00170.039 0.0033 0.109 — 0.095 834 ∘ Comparative steel M 0.079 0.03 0.960.013 0.0013 0.047 0.0041 0.069 Nb: 0.009 0.053 835 x Comparative steelN 0.037 0.03 0.68 0.012 0.0029 0.042 0.0045 0.076 — 0.056 860 ∘Comparative steel O 0.032 0.17 0.83 0.033 0.0045 0.033 0.0048 0.098 —0.075 864 ∘ Inventive steel P 0.034 0.22 0.84 0.033 0.0047 0.033 0.00550.105 — 0.079 867 ∘ Inventive steel Ti*= Ti—N × (48/14) − S × (48/32)Formula (1): Ti* ≧ 0.1, Formula (2): C × (48/12) − 0.14 < Ti* < C ×(48/12) + 0.08; C, S, N, and Ti in Formulae (1) and (2) denote theamounts (mass %) of the correspond *1“0” means that both Formulae (1)and (2) are satisfed. “X” means that one or both of Formulae (1) and (2)are not satisfied.

TABLE 2 Manufacturing conditions for hot-rolled steel sheet Finish-Total reduction Soaking Hot-rolled Plating Sheet Heating rolling ratioat last Average Coiling temperature steel treatment thicknesstemperature temperature two finish cooling rate temperature forannealing sheet No. Steel *2 (mm) (° C.) (° C.) rolling passes (%) (°C./s) (° C.) treatment (° C.) Remarks  1 A — 3.2 1180 900 55 70 700 —Example  2 B — 2.6 1250 950 50 80 645 — Example  3 B GI 2.6 1250 915 4595 590 695 Example  4 B GI 2.6 1250 920 45 85 595 785 Comparativeexample  5 C — 2.9 1200 890 40 60 560 — Example  6 D — 2.0 1220 935 50105  640 — Example  7 D — 2.0 1220 840 60 130  650 — Comparative example 8 D — 2.0 1220 905 75 100  660 — Comparative example  9 E GA 3.2 1240910 60 95 630 645 Example 10 E GA 3.2 1240 915 50 85 455 660 Comparativeexample 11 E GA 3.2 1240 920 50 30 660 715 Comparative example 12 F —2.3 1280 900 40 115  635 — Example 13 G — 2.0 1290 920 35 140  630 —Example 14 H — 4.5 1250 865 55 50 695 — Example 15 I — 3.4 1270 890 4585 575 — Example 16 J GA 2.0 1260 910 50 70 510 740 Example 17 K — 2.61250 900 55 75 595 — Comparative example 18 L GA 2.9 1250 900 40 80 560700 Comparative example 19 M GA 2.9 1200 880 45 45 590 685 Comparativeexample 20 N — 2.6 1250 910 60 65 690 — Comparative example 21 O — 2.01220 935 50 105  640 — Example 22 P — 2.0 1220 935 50 105  640 — Example*2 “—” represents no plating treatment. “GI” represents hot-dipgalvanizing treatment. “GA” represents hot-dip galvanizing treatment andalloying treatment.

TABLE 3 Microstructure of hot-rolled steel sheet Number percentage ofcarbide having grain Percentage of Mechanical properties of hot-rolledsteel sheet Hot-rolled Ferrite phase size of less than 9 Ti precipitateTensile Total steel fraction nm out of carbide having grain strengthelongation Burring sheet No. (area %) *3 containing Ti (%) size of 20 nm(%) TS (MPa) El (%) ratio (%) Remarks  1 91(P: 9) 70 85 545 34 70Example  2 97(P: 3) 85 80 563 33 81 Example  3  98(B: 2) 75 50 568 31 82Example  4    92(P: 5, B: 3) 55 40 525 24 46 Comparative example  5   91(P: 2, B: 7) 75 55 593 30 73 Example  6 98(P: 2) 90 85 631 33 88Example  7 91(P: 9) 40 40 530 23 31 Comparative example  8 93(P: 7) 5050 533 21 37 Comparative example  9 97(P: 3) 85 75 633 32 84 Example 10     91(B: 5, M: 4) 65 35 534 21 46 Comparative example 11  79(P: 21) 2550 516 23 43 Comparative example 12 92(P: 8) 85 80 564 33 67 Example 1398(P: 2) 80 85 689 25 65 Example 14 94(P: 6) 70 80 604 32 77 Example 15 99(B: 1) 75 65 699 26 75 Example 16    95(P: 1, B: 4) 80 50 678 22 80Example 17 95(P: 5) 70 40 533 23 44 Comparative example 18  93(B: 7) 7035 642 18 25 Comparative example 19     55(P: 40, B: 5) 25 40 529 17 28Comparative example 20 96(P: 4) 55 55 581 25 47 Comparative example 2198(P: 2) 90 45 605 34 63 Example 22 98(P: 2) 90 40 594 32 60 Example *3The percentages of microstructures other than ferrite phase are shown inparentheses. P: pearlite (including cementite), B: bainite, M:martensite (including retained austenite).

The hot-rolled steel sheets according to examples (Nos. 1 to 3, 5, 6, 9,12 to 16, 21, and 22) had the desired tensile strength (540 MPa or more)and good mass production burring formability. By contrast, thehot-rolled steel sheets according to comparative examples (Nos. 4, 7, 8,10, 11, and 17 to 20), which were outside the scope of the presentinvention, did not have a desired high strength or sufficient burringratios.

1. A high-strength hot-rolled steel sheet comprising, on a mass percentbasis: C: 0.013% or more and less than 0.08%, Si: less than 0.5%, Mn:more than 0.8% and less than 1.2%, P: 0.05% or less, S: 0.005% or less,N: 0.01% or less, Al: 0.1% or less, and Ti: 0.03% or more and 0.15% orless such that C, S, N, and Ti satisfy the following formulae (1) and(2), the remainder being Fe and incidental impurities, wherein thehigh-strength hot-rolled steel sheet has a microstructure in which aferrite phase fraction is more than 90%, a carbide containing Ti isprecipitated, and 70% or more of the carbide has a grain size of lessthan 9 nm:0.05≦Ti*<0.1  (1)C×(48/12)−0.16<Ti*  (2) wherein Ti*=Ti−N×(48/14)−S×(48/32), and C, S, N,and Ti denote the amounts (% by mass) of the corresponding elements. 2.The high-strength hot-rolled steel sheet according to claim 1, wherein50% by mass or more of Ti is precipitated as precipitates containing Tihaving a grain size of less than 20 nm.
 3. The high-strength hot-rolledsteel sheet according to claim 1, further comprising at least one of V:0.002% or more and 0.1% or less and Nb: 0.002% or more and 0.1% or lesson a mass percent basis.
 4. The high-strength hot-rolled steel sheetaccording to claim 1, further comprising at least one of Cu: 0.005% ormore and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.002%or more and 0.2% or less, and Mo: 0.002% or more and 0.2% or less on amass percent basis.
 5. The high-strength hot-rolled steel sheetaccording to claim 1, further comprising B: 0.0002% or more and 0.003%or less on a mass percent basis.
 6. The high-strength hot-rolled steelsheet according to claim 1, further comprising at least one of Ca:0.0002% or more and 0.005% or less and REM: 0.0002% or more and 0.03% orless on a mass percent basis.
 7. A method for manufacturing ahigh-strength hot-rolled steel sheet, comprising: heating steel having acomposition according to claim 1 to 1100° C. or more, hot-rolling thesteel at a finish-rolling temperature of (Ar₃+20° C.) or more and at atotal reduction ratio of 60% or less at last two finish rolling stands,cooling the hot-rolled steel sheet at an average cooling rate of 40°C./s or more, and coiling the hot-rolled steel sheet at a coilingtemperature in the range of 560° C. to 720° C.
 8. A method formanufacturing a high-strength hot-rolled steel sheet, comprising:heating steel having a composition according to claim 1 to 1100° C. ormore, hot-rolling the steel at a finish-rolling temperature of (Ar₃+20°C.) or more and at a total reduction ratio of 60% or less at last twofinish rolling stands, cooling the hot-rolled steel sheet at an averagecooling rate of 40° C./s or more, coiling the hot-rolled steel sheet ata coiling temperature in the range of 500° C. to 660° C., annealing thehot-rolled steel sheet at a soaking temperature of 750° C. or less afterpickling, and plating the hot-rolled steel sheet by immersing thehot-rolled steel sheet in a molten zinc bath.
 9. The method formanufacturing a high-strength hot-rolled steel sheet according to claim8, wherein the plating treatment is followed by alloying treatment.